The turbine is a well known apparatus which is characterized by a rotating member which turns in response to an applied force. Typically this force is generated by passing a high pressure fluid such as steam over a series of blades which are connected to the rotating member. The efficiency of a turbine depends on its ability to use as much of this applied force as possible to maintain rotation. Sealing devices maintain turbine efficiency by preventing steam from escaping from the working space of the turbine and by preventing air from leaking into the working space of the turbine.
The sealing devices used on turbines are known variously as steam seals, gland seals, labyrinth seals, steam packings, diaphragm packings, and pressure packings. A typical steam seal is shown in cross section in FIG. 1. In this illustration, a plurality of sealing strips 4 extend toward a rotor 2. The sealing strips 4 are preferably provided with a triangular or knife edge. The clearance between the sealing strips 4 and the rotor 2 is shown as the distance labeled C.sub.s. The sealing strips 4 are supported by a ring 6. The ring 6 is mounted to a stationary portion 8 of the turbine. A spring 10 pushes the ring 6 inward toward the rotor 2 and allows for radial expansion of the ring away from the rotor. A seal such as this would be located, for example, near a point where the rotor emerges through an opening in the casing of the turbine. In operation, the sealing strips serve to contain most of the fluid that would otherwise escape through the space between the rotor and the stationary portion of the turbine which surrounds it.
In some applications it is typical to use several rings 6, each supporting several sealing strips 4. FIG. 2 shows a cross section of a stationary member 8 of a turbine to which are mounted several rings 6, each ring supporting several sealing strips 4.
A perspective view of a section of a seal which would surround a rotor of a turbine is shown in FIG. 3. The seal includes a packing head 26 which in operation would be mounted to a stationary member of the turbine to partially encircle a rotating member of the turbine. Several ring segments 28 are mounted in slots 29 provided in the packing head. Each ring segment 28 supports several sealing strip segments 30. The illustrated section may be joined with a complimentary section to encircle and seal a rotating member of a turbine.
FIG. 4 illustrates several ring segments 28 which would be used in a packing head such as that illustrated in FIG. 3. Each ring segment supports several sealing strip segments 30. The sealing strip segments define an interior radius R.sub.SEAL. This radius is chosen to correspond to a portion of a rotating member of a turbine so as to provide a desired clearance when placed in a sealing arrangement with the rotating member.
FIG. 5 shows a seal which is used with a rotating member 2 which has alternating high portions 12 and low portions 14 on its surface. The sealing strips 4 are chosen to define interior radii R.sub.SEAL such that a clearance C.sub.s is maintained at the tip of each sealing strip 4 between the sealing strip 4 and its corresponding portion of the rotor 2.
The effectiveness of steam seals depends significantly on maintaining a minimal clearance between the sealing strips and the rotating member. Contact between the sealing strips and the rotating member will wear the sealing strips, thereby increasing clearance and decreasing the efficiency of the seal. Contact is most likely to occur as a result of transient conditions, during which the rotating member may be displaced from its normal position relative to the seal. These conditions often coincide with the starting or stopping of the turbine, load rejections, or overspeeds. Transient conditions may cause the rotating member to rub against the seal, wearing the sealing strips and leaving rubs on the surface of the rotating member. It is therefore desirable to prevent contact between the rotating member and the sealing strips.
Additional useful background information may be found in Sanders, Turbine Steam Path Engineering for Operations & Maintenance Staff, 1988, the contents of which are incorporated herein by reference.